1. Field of the Invention
The present invention relates, in general, to manufacturing methods of laminated polar anisotropic hybrid magnets having higher magnetic properties than those of conventional anisotropic injected magnets, which can reduce the use of expensive magnet materials. More specifically, the present invention relates to a method of manufacturing a laminated polar anisotropic hybrid magnet, characterized in that an inexpensive permanent magnet powder having low magnetic properties and an expensive permanent magnet powder having high magnetic properties are separately mixed with a thermoplastic resin, after which first and second injection molding processes are sequentially performed in a magnetic field by use of polar anisotropic molds having different outer diameters. In such cases, as for manufacturing a ring-type anisotropic bonded magnet by means of an injection molding process or a compression molding process, the permanent magnet materials are layered and hybridized using magnetic circuit design technique, whereby the above anisotropic bonded magnet can be economically increased in magnetic properties, and has magnetic flux density waves controllable on the magnet surface to obtain flux density waves suitable for performances and characteristics of motors, with enhanced temperature properties. Hence, the manufacturing method of the laminated polar anisotropic hybrid magnet of the present invention is efficiently improved, resulting in increased productivity and reliability in practical use thereof.
2. Description of the Related Art
In recent years, permanent magnets having high magnetic properties are increasingly required to be manufactured, according to development of design techniques for products, such as motors, actuators, or medical instruments, as well as miniaturization and high functionality of constitutive parts and materials used for such products.
The permanent magnets having high magnetic properties are mainly applicable for high-powered motor products, for example, VCRs, laser printers, hard disk drives (HDD), robots, electric power steering, automobile fuel pumps, washing machines, refrigerators, air conditioners, etc. As such, high magnetic properties of the permanent magnet result in variously changeable motor design techniques and wide applications thereof. Further, the sizes of end products can be decreased, and thus, manufacturing costs thereof are lowered. Additionally, by utilizing motors having high efficiencies, energy saving of end products can be expected. Thus, main research for permanent magnets intends to develop a permanent magnet material having high energy product, or to maximize a surface flux density of the permanent magnet by optimizing the magnetic circuit designs even though the same permanent magnet material is used.
Although the former research requires high material costs for the development of the magnet materials, the latter research may economically be undergoing because the magnetic properties may increase by means of only the magnetic circuit design technique.
Typically, conventional methods of manufacturing a ring-type anisotropic bonded magnet are classified into an injection molding process, and a compression molding process.
As for the injection molding process, permanent magnet powders for the bonded magnet, such as ferrite powders, alnico powders, Sm—Co powders, HDDR (Hydrogen Disproportionation Desorption Recombination)-treated Nd—Fe—B powders, Sm—Fe—N powders, etc., are mixed with a thermoplastic resin (e.g., nylon) at 150-300° C. in an atmosphere or inert gas, to prepare a compound, which is further heated at 150-300° C. to have flowability, followed by injection molding by use of a mold having a predetermined cavity which is applied with a magnetic field.
As for the compression molding process, permanent magnet powders for the bonded magnet, such as ferrite powders, alnico powders, Sm—Co powders, HDDR-treated Nd—Fe—B powders, Sm—Fe—N powders, etc., are mixed with a thermosetting resin, for example, epoxy, in the temperature range of room temperature to 100° C. in an atmosphere or inert gas, to prepare a compound, which is then placed into a mold having a predetermined cavity, and thereafter applied with a magnetic field, whereby the compound is oriented in the magnetic field direction and compressed.
In manufacturing the ring-type anisotropic bonded magnet, when the compound is charged in the mold which is applied with the magnetic field by a permanent magnet or an electromagnet, the magnet powders of the compound are oriented in the magnetic field direction. As such, as shown in FIG. 1a, a magnetization direction of the magnet is formed in a radial direction (arrow direction) facing outward from the center of the circle. Such a magnet is called a radial magnet 10, which has a surface flux density formed in a saw-toothed wave shape along the circumference of the radial magnet 10.
The radial magnet 10 has excellent magnetic properties and is used to form an integrated ring magnet, thus generating economic benefits, compared to a ring magnet obtained by assembling C-shaped magnet parts. However, since the radial magnet 10 has the surface flux density having a saw-toothed wave shape, a magnetic force between the magnet and the silicon steel plate of the armature in the motor becomes high, therefore causing a cogging phenomenon.
In addition, FIG. 1b shows an orientation direction of magnetic field of a polar anisotropic magnet 20 which is distributed in only the outside of the circle. Compared to the radial magnet having the same pole numbers and sizes manufactured by use of the same permanent magnet material, the above polar anisotropic magnet 20 is higher by 30-40% in surface flux density and has a sinusoid suitable for use in the motor. However, the anisotropic magnet 20 is disadvantageous in terms of high manufacturing costs, due to requirement of extra magnet materials for the formation of the magnetic path up to the inner part of the magnet.
With the intention of increasing the magnetic properties (surface flux density) of the ring-type anisotropic bonded magnet, a volume ratio of the magnet powder in the compound increases, or a rare earth powder, such as Sm—Co powder, HDDR-treated Nd—Fe—B powder, Sm—Fe—N powder, etc., is further used. However, the rare earth powder is about 10 times as expensive as the ferrite powder having low magnetic properties. Hence, the rare earth powder is restrictedly utilized in only the motor requiring high characteristics.
Alternatively, to manufacture the ring magnet having desired magnetic properties while reducing material costs, the ferrite powder and the rare earth powder are mixed at a proper ratio upon a compounding process, thereby obtaining a polar anisotropic hybrid magnet 30, as shown in FIG. 2a. Such a polar anisotropic hybrid magnet 30, composed of ferrite and rare earth powders mixed at 50:50 vol %, has a surface flux density in proportion to the volume ratio of the rare earth powder, and hence, it cannot function desirably, thus negating economic benefits (FIG. 2b).
Meanwhile, the ring-type anisotropic bonded magnet having high magnetic properties results from the use of the rare earth powder having high magnetic properties, such as HDDR-treated Nd—Fe—B powder, Sm—Fe—N powder, etc. However, coercive force of the above anisotropic bonded magnet drastically decreases at a rate of −0.4 to −0.45%/° C. and −0.4 to −0.42%/° C., according to the temperature increase. Therefore, the above anisotropic bonded magnet is lower in thermal reliability for magnetic performance, compared to a bonded magnet prepared by using the relatively inexpensive ferrite powder (coercive force change: 0.35-0.55%/° C.), and cannot be applied for motors employed at relatively high temperatures.
Consequently, conventional manufacturing methods of the ring-type anisotropic bonded magnet are limited in efficiencies thereof, thus remarkably decreasing productivity and reliability in practical use thereof.